The next-generation mobile communication system must be able to transmit high quality and high capacity multimedia data at high speed using limited frequency resources. In order to enable it, inter-symbol interference and frequency selective fading, which occur at the time of high-speed transmission, must be overcome while maximizing frequency efficiency.
In order to improve the performance of the mobile communication system, a closed loop transmission scheme employing channel response information between a Base Station (BS) and a User Equipment (UE) has emerged. An Adaptive Modulation and Coding (AMC) scheme is the technology of increasing the link performance by controlling the modulation and coding method in a BS using feedback channel response information. Alternatively, the channel response information can be used for a BS to pre-process a channel so as to reduce a complex equalization process in a UE.
A method of obtaining downlink channel information is based on feedback information sent by a UE. The UE measures downlink channel response and constructs it as adequate feedback information. A BS that has received the feedback information performs downlink data scheduling using the feedback information. For example, the feedback information may have a quantization type of measured channel response.
In contrast, there are two types of reference signals transmitted over the uplink. One is a reference signal for estimating a channel in order to demodulate the uplink data, and the other is a reference signal for scheduling the frequency domain of the uplink by confirming the channel state of the uplink. The former is also called a data demodulation (DM) reference signal and the latter is also called a scheduling reference signal or a sounding signal. The DM reference signal is transmitted in a limited frequency domain only when there exists data transmitted over the uplink, whereas the scheduling reference signal is transmitted periodically over the entire frequency bands of the uplink irrespective of the existence of data.
The UE sends the scheduling reference signal through the uplink channel, and the BS confirms the channel state based on the scheduling reference signal and then performs scheduling for uplink transmission.
The scheduling reference signal transmitted over the downlink within one cell is broadcasted by the BS, whereas the scheduling reference signal transmitted over the uplink within one cell is transmitted from the entire UEs, existing in the cell, to the BS over the entire frequency bands. If a plurality of UEs transmits the scheduling reference signals over the entire frequency bands in a radio communication system with limited uplink frequency resources, interference occurs between the scheduling reference signals transmitted by the respective UEs. Thus, there is a need for a method of transmitting the scheduling reference signals efficiently.